A combine harvester with a multi-drum threshing system has been described in the German Patent DE 30 42 734 C. The mixture of grain and chaff passing through openings in the threshing concave and the downstream separating baskets is fed via grain pans to a cascade pan to which a rearwardly directed current of air is applied via an upper fan outlet. Disposed below this cascade pan is a precleaning sieve to which a current of air is also applied via the upper fan outlet, and disposed below said precleaning sieve is an additional grain pan. The rearwardly moving mixture on the upper surface of the precleaning sieve falls on an upper sieve, while the precleaned grain and chaff which fall downwardly through the precleaning sieve are transported via the additional grain pan to a lower sieve. A rearwardly directed current of air is applied to the upper and lower sieves by means of a lower fan outlet.
The disadvantage of this design is that the precleaned mixture of grain and chaff which has been passed through the additional grain pan to the lower sieve is compacted to a relatively high degree since it was passed only through the precleaning sieve and was subsequently transported through the additional grain pan. A cascade pan is, in fact, disposed between the additional grain pan and the lower sieve, but only a relatively low current of air is applied to said cascade pan. Thus, compacted material falls onto the lower sieve, which, given a certain sieve opening, limits the potential throughput through the lower sieve and thus through the combine harvester. It is possible to further open the fins of the lower sieve in order to increase the throughput; however, this has a detrimental effect on the cleanness of the ultimately harvested grain.